Expanding the Rich Crystal Chemistry of Ruthenium(V) Oxides via the

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Expanding the rich crystal chemistry of ruthenium (V) oxides via the discovery of BaRuO, BaRuO , BaRuO and SrRuO(OH) by pH controlled hydrothermal synthesis 2

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Thomas Marchandier, Quentin Jacquet, Gwenaëlle Rousse, Benoît Baptiste, Artem M. Abakumov, and Jean-Marie Tarascon Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02510 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 28, 2019

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Chemistry of Materials

Expanding the rich crystal chemistry of ruthenium (V) oxides via the discovery of BaRu2O6, Ba5Ru4O15, Ba2Ru3O10 and Sr2Ru3O9(OH) by pH controlled hydrothermal synthesis

Thomas Marchandier1,2,3, Quentin Jacquet1,2,3, Gwenaëlle Rousse1,2,3, Benoît Baptiste3,4, Artem M. Abakumov5, and Jean-Marie Tarascon1,2 1. Collège de France, Chaire de Chimie du Solide et de l’Energie, UMR 8260, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France 2. Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, 75005 Paris, France 3. Sorbonne Université, 4 place Jussieu, F-75005 Paris, France 4. Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, CNRS, UMR 7590, 4 Place Jussieu, 75005 Paris, France 5. Center for Energy Science and Technology, Skolkovo Institute of Science and Technology, 3 Nobel Street, Moscow, 143026, Russia

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Abstract: Along the years ruthenium oxides have been deeply investigated for their magnetic or catalytic properties. The exploration of new synthesis processes, and especially low temperature ones, is of primary importance to obtain new materials with interesting properties. Here we highlight the tunability of a low temperature (200°C) hydrothermal synthesis route of alkaline-earth ruthenates. Playing only with simple physico-chemical parameters such as pH, it is possible to obtain a large diversity of metastable compounds. Among them four compounds, namely BaRu2O6, Ba2Ru3O10, Ba5Ru4O15 and Sr2Ru3O9(OH), are reported here for the first time. The influence of the reaction parameters (temperature, counter ions, reactant ratio and pH) is studied. According to these observations, the importance of the reaction pH is highlighted and a reaction scenario is proposed. Finally the crystal structures of Ba2Ru3O10, Ba5Ru4O15 and Sr2Ru3O9(OH) are reported. These findings further highlight the richness of low temperature chemistry in the discovery of new metastable phases to be further explored by scientists.


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Introduction: Ruthenium oxides are widely studied in several research fields due to their panoply of unique electrical and magnetic properties. Among them, ruthenium dioxide (RuO2) is known to be the most efficient electrocatalyst for water splitting1. Besides, the binary ruthenium oxides are of prime importance. Recently, layered alkali ruthenium oxides such as Li2RuO3 have been used within the field of energy storage, as model insertion compounds, to unravel the electrochemical activity of the anionic framework in Li-rich layered oxides electrodes for the next generation of Li-ion batteries2. Among all the binary combination, alkaline-earth ruthenates are the most studied for their magnetic properties. For instance, Sr2RuO4 sparked considerable attention for its unconventional spin-triplet superconductivity3 and more recently SrRu2O6 has been found to be an antiferromagnet with superhigh Néel temperature (565 K)4. Owing to such a continuous interest, there is a need to enlarge the Ru-based oxides family members via additional chemical exploration. Until now ruthenium-based oxides have been prepared via ceramic processes enlisting high temperature (or high pressure) to facilitate the diffusion of chemical elements in the solid state. These modus operandi present the advantages of reducing the number of steps, of co-reactants and allow the production of large amount of pure crystalline products with high reproducibility. Nevertheless, these drastic conditions lead solely to the thermodynamically stable compounds. In order to develop more “eco-friendly” processes and enable the synthesis of metastable phases, lowtemperature processes (sol-gel, hydrothermal, co-precipitation etc…) have been developed over the last fifty years. These low temperature synthesis approaches, also referred as "chimie douce", involve multiple chemical steps as oxydoreduction, proton transfers, and/or cluster formation. Although such reactions proceed via complex nucleation-growth mechanisms, they offer many opportunities to tune the reaction pathway and obtain different materials playing with physico-chemical tools (pH, temperature, reactant ratio…). The literature is rich of elegant studies showing how the control of solvated species or reaction kinetics steps can lead to the oriented synthesis of vanadium binary oxides5 or the polymorphic control of TiO26 or FeS27, to name only a few. In light of the benefits provided by the "chimie douce", some authors have decided to implement it to the synthesis of ruthenium oxides with high ruthenium redox state (greater than IV). The hydrothermal approach with either high temperature or pressure was first tried but success had been limited 8,9 to the synthesis of three new metastable ruthenium(V)/alkaline-earth oxides by Hiley et al. in 2014 using mild temperature synthetic conditions (200°C).10 The novelty in their approach relies in the use of a highly oxidized ruthenium salt (KRuVIIO4) which is reduced during the reaction, rather than in the oxidation of a ruthenium precursor having a low (+IV) oxidation state as previously. Following this work several other ruthenium (+V) phases were obtained using the same chemical trick.11,12,13 Surprisingly, none of these studies went into a deep understanding of the reacting paths



by which these new phases were formed. Progress in this direction, however, cannot be uncoupled from a deeper appreciation of the means by which the reduction and precipitation steps are affected by various physico-chemical parameters. Here we tackle this issue and use our findings to unravel new phases. Starting from KRuVIIO4 and alkaline-earth (M) salts (BaCl2, Sr(NO3)2) we show that the modification of simple physico-chemical parameters such as reactant ratio, temperature and principally pH leads to the formation of a large diversity of metastable ruthenium(+V, +VI) binary oxides. The link between these experimental parameters and the modification of the reaction pathways is discussed as well. Among the prepared phases, the four compounds BaRu2O6, Ba2Ru3O10, Ba5Ru4O15 and Sr2Ru3O9(OH) have been obtained for the first time and the structure of three last ones is reported here.

Experimental Section Synthesis For all the synthesis described in the following sections, we used KRuO4 (Alfa Aesar), BaCl2.2H2O (Alfa Aesar), and Sr(NO3)2 (Sigma Aldrich) as sources of Ru, Ba and Sr, respectively. To prepare the aqueous alkaline solutions, extra pure KOH (>99.98 metal basis, Alfa Aesar) was used, as well as NaOH (98% Metal basis, Sigma-Aldrich) and LiOH (98% Metal basis, Alfa Aesar). Our synthesis procedure throughout this manuscript, if not otherwise mentioned, consists in i) mixing of 5 mg (2.45x10-2 mmol) of KRuO4 with the desired amount of alkaline-earth salt in 1 mL of aqueous solution of alkaline hydroxide with controlled concentration, ii) pouring in air the mixtures into a hermetically sealed tailor made 2 mL Teflon-lined steel autoclave and iii) placing the autoclave at 200°C in a preheated chamber furnace for 72 hours. After the reaction, powders are cleaned three times: twice with 5 mL of a 10-2 M solution of HCl and then with water before being dried at 100°C overnight. The resulting powder was then characterized for phase purity and composition. X-ray Diffraction (XRD) Synchrotron X-ray diffraction measurements (XRD) were performed on the 11-BM beamline of the Advanced Photon source at Argonne National Laboratory, with a wavelength at 0.413 Å (the exact wavelength for each materials is given with the refinement). Laboratory powder XRD measurements were performed with a Bruker D8 Advance diffractometer operating in the Bragg-Brentano geometry at with Cu Kα radiation (λ(Kα1) = 1.54056 Å, λ(Kα2) = 1.54439 Å) and a Lynxeye XE detector. Rietveld refinements were performed using the FullProf program14. Single crystal X-ray diffraction (SCXRD) data were collected at the X-ray diffraction platform of IMPMC, on a Rigaku MM007HF diffractometer equipped with a RAXIS4++ image plate detector, a Mo


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rotating anode (λ = 0.71073 Å, Varimax multilayer optics) at 293 K. Following an equivalent procedure as described by A. Rothkirch et al.,

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the collected images were converted to the

esperanto format with an in-house program (not published). Then, data reduction, cell refinement, space group determination, scaling and empirical absorption correction were performed using CrysAlisPro software( CrysAlisPro 1.171.38.46, Rigaku Oxford Diffraction, 2015). The structure was solved using SHELXT16 implemented in Olex2 program17. The refinement was then carried out with SHELXL, by full-matrix least squares minimization and difference Fourier methods. All atoms were refined with anisotropic displacement parameters. Scanning Electronic Microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) Scanning electronic microscopy and EDX analysis were performed using an SEM-FEG Hitachi SU-70 microscope coupled with an Oxford X-Max 50 mm2 energy dispersive X-ray (EDX) spectrometer. TGA analysis Thermogravimetric analysis (TGA) was performed using a Metler Toledo TGA/DSC 3+ (LF1100°C) under argon atmosphere in order to determine the changes in sample weight with increasing temperature in order to evaluate the decomposition temperature of the different compounds synthetized. For this, a heating ramp from 25°C to 800°C was imposed, with a heating rate of 5°C per minute. Transmission electron microscopy (TEM) TEM sample was prepared in air by crushing the crystals in a mortar in anhydrous ethanol and depositing drops of suspension onto holey carbon grids. Electron diffraction (ED) patterns, high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images and atomic resolution EDX maps were obtained with an aberration-corrected Titan G3 electron microscope equipped with a Super-X EDX system and operated at 200 kV.

Results and discussion 1) Synthesis As mentioned earlier the goal of this study is to investigate the hydrothermal synthesis of ruthenium oxides proposed by Hiley et al.10 and see how it can be influenced by acting on physico-chemical parameters. The synthesis consists in reacting a ruthenium (VII) salt (KRuO4) with alkaline-earth (BaCl2, Sr(NO3)2) salts in aqueous alkaline hydroxides solution (KOH, NaOH or LiOH) at 200°C. Hydrothermal syntheses being known as extremely sensitive to physico-chemical parameters such as



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pH, reactant ratio, cations in solution or temperature, a survey of these various parameters has been undertaken.

Impact of KOH concentration and reactant ratio: speciation diagram For guidance purpose speciation diagrams were first experimentally drawn (Figure 1) as function of the KOH concentration and of the MX2/KRuO4 ratio (with MX2 = BaCl2 or Sr(NO3)2). To build these diagrams, three different molar ratios of BaCl2/KRuO4 reactants (0.5, 1 and 2) were studied. For each of these ratios, nine concentrations of KOH ranging from 0 to 8 mol.L-1 were considered. The same ratios were used for Sr(NO3)2/KRuO4, with seven concentrations of KOH ranging from 0 to 3 mol.L-1. We observed that the higher KOH concentration is, the lower the amount of powder is formed at the end of the reaction (after 72 hours) and this also depends on the nature of alkaline earth cation. As a consequence, no KOH] concentration greater than 8 M and 3 M will be studied from now on for the barium/ruthenium and strontium/ruthenium systems, respectively. The resulting samples from such a survey that correspond to symbols in the diagram (Figure 1) were analysed for phase purity by XRD and single phase domains are defined by different colours. For multiphase samples, the relative ratios are not given, as they slightly fluctuate from one experiment to the other. For the barium/ruthenium system with a BaCl2/KRuO4 ratio of 1 we observed the formation of five phases (Figure 1a) upon increasing KOH concentration: BaRu2O6 ([KOH]

Expanding the Rich Crystal Chemistry of Ruthenium(V) Oxides via the

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